Reference is made to U.S. Pat. No. 4,759,280 to Malashenko which describes a hydraulic press having a stationary lower platen, a movable upper platen, and a number of long-stroke, light-duty hydraulic cylinders for general displacement and positioning of the upper platen. A short-stroke, large-bore hydraulic ram travels with the upper platen. A gate mechanism is closed above the ram just before compression phases of operation commence and the ram is then used to drive the upper platen downwardly. This arrangement has several advantages over more conventional presses in which a large central ram is attached both to the upper platen and to a cross-head of the press. These include reduced cycle time. Also, the stroke of the ram can be limited to considerably less than a meter, sufficient for compression phases of operation, thereby increasing the response time of the ram and permitting more precise control of the compression operation.
The prior patent also addresses the problem of adjusting the clearance between the upper and lower platens when the gate mechanism is closed. In molding applications, for example, the clearance may have to be adjusted to accommodate molds of different height. Increasing the stroke of the ram to accommodate a wide range of possible clearances is undesirable. The capacitance of the ram increases and its speed of response decreases. The prior patent teaches use of spacers stacked on the upper end of the ram to adjust platen clearance. Removing a spacer increases platen clearance; adding a spacer decreases platen clearance. In presses intended for compression molding of plastic products, such spacers may weigh several hundred kilograms and may be manipulated with a small hoist. This is acceptable in many installations, where molds are changed relatively infrequently. However, it would be desirable to simplify the handling of such spacers and particularly to reduce the time required to adjust platen clearance and change molds for production of alternative products.